The present invention relates to a method for dehydrohalogenating a halogenated hydrocarbon by contacting the compound with a solution containing magnesium hydroxide at high temperatures and pressures.
Polychlorinated ethanes can all be dehydrochlorinated to corresponding chloroethylenes which are used commercially. For example, 1,2-dichlorethane yields vinyl chloride, the well-known monomer for the production of polyvinylchloride; the tetrachloroethanes yield trichloroethylene, a widely used industrial degreasing solvent; and pentachloroethane yields perchloroethylene, the widely used nonflammable dry-cleaning solvent. Depending on the dehydrohalogenation method, 1,1,2-trichlorethane may yield all three isomeric dichloroethylenes or predominantly only one of the dichloroethylenes. Of the three isomers only 1,1-dichloroethene (vinylidene chloride) has found wide use. In addition to its use as a monomer for polymer production, it has also become an intermediate for the production of herbicides, latexes, and 1,1,1-trichloroethane which is used as a degreasing solvent. Therefore, the method which yields predominantly vinylidene chloride is preferred.
The classical method of dehydrochlorinating the polychloroethanes employs an aqueous suspension of alkaline earth hydroxides such as calcium or barium hydroxide. In general, these reactions are run under ambient pressures at a temperature at or below the boiling point of the chlorinated reactant. For example, U.S. Pat. No. 2,598,646 discloses a continuous process for dehydrochlorinating a polychloroethane by contacting an aqueous alkaline earth metal hydroxide slurry in a closed chamber with a polychloroethane at a reaction temperature below the boiling point of any of the organic azeotropes formed therein. While the examples report good yield using calcium hydroxide, data for magnesium hydroxide is not provided. It is known, however, that the process produces poor conversions to vinylidene chloride when magnesium hydroxide is used as the alkaline earth metal hydroxide in such a process.
More recently, methods employing an aqueous solution of an alkali metal hydroxide dispersed in the organic chloroethane phase by means of powerful mixing devices have found widespread use. These reactions are typically run at or below the boiling point of the chlorinated reactant and under ambient pressures. In these cases, extreme care must be taken to avoid an excess of the hydroxide. Otherwise, the vinylidene chloride product can undergo dehydrochlorination to hazardous acetylenic compounds. Due to the potential of producing acetylenic compounds, less than stoichiometric amounts of alkali metal hydroxides are typically used.
Therefore, the existing problem is that the use of magnesium hydroxide by itself has been unsuccessful due to the poor solubility of the hydroxide ion. Due to the widespread availability, low cost, and other advantages described herein, it would be advantageous to the chemical industry and to the public to possess a method whereby magnesium hydroxide could be used to dehydrohalogenate halogenated organic compounds. Such a process would provide another route to vinylidene chloride when, for example, the price of caustic is high. It would also be advantageous to have a method whereby the product can be selectively formed without precise equivalent mixtures of reactant and base, and wherein the formation of potentially explosive acetylene compounds does not occur.